The present invention generally relates to an internal combustion engine and more particularly, to a camshaft driving device for use in an internal combustion engine having a reduction gear therein, which can be increased in rigidity and whereby the engine can be formed into a compact size.
As one of the methods for driving one or more camshafts of an internal combustion engine, there has generally been widely employed such a driving system in which a camshaft or camshafts are directly driven by a driving shaft through an endless chain or an endless belt which is passed around therebetween.
In a driving system of this kind for directly driving the camshafts, however, since there is necessarily arranged on each camshaft, a gear having a diameter two times larger than that of a gear arranged on the driving shaft in terms of the reduction gear ratio, the engine is undesirably increased in size in the radial direction of the driving shaft.
Accordingly, in order to eliminate the aforementioned drawback and to cause the engine to be compact, there has been proposed a camshaft driving system as disclosed, for example, in Japanese Utility Model Laid-Open Publication Jikkaisho No. 59-35603, wherein a reduction gear is disposed between a driving shaft and camshafts so as to transmit a driving force from the driving shaft to the camshafts through the reduction gear and chains or belts, and this results in that the gears such as pulleys or sprockets of the camshafts can be restricted to be small in diameter.
In the camshaft driving system for use in the internal combustion engine having the aforementioned reduction gear therein, however, since the reduction gear receives not only a fluctuating torque produced by an explosion force from the driving shaft, but also that produced by a cam lift from the side of the camshafts, a large amount of load acts on a supporting portion for the reduction gear. Nevertheless, when the supporting portion for the reduction gear is reinforced on a large scale, the engine in inevitably increased in weight.
Furthermore, since such reduction gear is composed of a pair of gears e.g. timing pulleys or sprockets which are integrally formed together with a shaft disposed therebetween in the axial direction of the driving shaft, there arises such a drawback that the engine is undesirably increased in size in the axial direction of the driving shaft owing to the arrangement of the reduction gear.